World can't afford new Gulf war, warns UN chief
The latest incident, the second in the Gulf of Oman in a month, came amid spiralling tensions between Tehran and Washington, which has pointed the finger at Iran over earlier tanker attacks in May.
At the request of the US, the Security Council was to meet behind closed doors later Thursday to discuss the situation in the Gulf.
Guterres said he strongly condemned any attack against civilian vessels.
"Facts must be established and responsibilities clarified," the UN secretary-general told a council meeting on UN cooperation with the Arab League.
"If there is something the world cannot afford, it is a major confrontation in the Gulf region."
US acting Ambassador Jonathan Cohen said it was "unacceptable for any party to attack commercial shipping," stopping short blaming Iran.
"Iran remains the most significant threat to regional peace and security, engaged in numerous malign activities in the region," he told the council.
The Norwegian Maritime Authority said three explosions were Thursday reported on board the Norwegian-owned tanker Front Altair after it was "attacked" along with the Singapore-owned ship Kokuka Courageous.
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Iran said its navy had rescued 44 crew members after the two vessels, which were carrying highly flammable material, caught fire.
Start a fire
Arab League Secretary-General Ahmed Aboul Gheit warned both the targeting of oil tankers and attacks against Saudi Arabia were "dangerous developments".
Yemen's Iran-aligned Houthi rebels said they had fired missiles on an airport in southwestern Saudi Arabia on Wednesday.
"Some parties in the region are trying to instigate fires in our region and we must be aware of that," Gheit told the council.
He urged the UN body to "act against those responsible to maintain security and stability in the region."
On 12 May, four oil tankers - two Saudi, one Norwegian and one Emirati - were damaged in still unexplained attacks in the Gulf of Oman off the UAE.
Abu Dhabi last week said initial findings of its investigation pointed to the likelihood that a "state actor" was behind the bombings, but did not specifically blame Iran.
The preliminary findings were presented to the council which may consider a response.
These showed that it was "highly likely" that four Limpet mines, which are magnetically attached to a ship's hull, were used in the attacks, placed by trained divers who were deployed from fast boats.
US National Security Advisor John Bolton has said "there's no doubt" that Iran was responsible for last month's attacks on the tankers.